IR Theory


The scanning infrared spectrometer measures the amount of light absorbed by a small portion of sample that is sandwiched between two NaCl windows. The light comes from an IR light source that is tuned to produce light at specific wavelengths throughout the IR spectrum. Note in the figure above how the light is split into two different beams by the partial mirror. One of the beams passes through the sample while the other is unobstructed. Consequently, the unobstructed beam is more intense than the beam that passed through the sample.

Both light beams strike a detector that is sensitive to IR radiation. Electronics within the spectrophotometer compare the two beams and determine the difference in light intensity. This difference is a measure of the light that has been absorbed by the sample. (i.e. if there was no sample present, the two light beams would be identical and their difference would be zero, that is, no light absorbed)

Unlike the Spectronic 20 visible spectrophotometer you may be familiar with, the scanning IR spectrophotometer operates over a range of wavelengths. Motors within the instrument automatically change the wavelength of light that passes through the sample while a chart recorder measures the amount of light absorbed. The graph produced is called a spectrum.

There are two different systems used to describe the interaction of light and sample. The first is the % transmission (%T) scale. This scale ranges from 0% to 100 % and indicates what percent of the original light actually gets through the sample. In other words, 25 %T means that the transmitted light was one quarter the intensity of the light originally striking the sample.

Alternately, absorbance (A) is a measure of how much light is absorbed by the sample. This system is related to %T in the following way:

In this equation we also establish the amount of light absorbed by a given sample depends on the concentration (C) of the sample where "l" is the sample thickness and E is a constant known as the extinction coefficient.

In the organic laboratory we will be less interested in determining solution concentrations from spectral data and more concerned with the appearance of the spectrum and what it tells us of functional groups and the compound's identification.